This is AS Biology Coursework for the OCR syllabus. I would really appreciate some clarification!

You are required to plan an investigation to find the lowest temperature that kills all the yeast cells in a suspension of either dried or fresh baker's yeast.

Your planning must be based on the assumption that you are provided with the following:
10% suspension of yeast
10% solutions of sucrose or glucose
1.0% methylene blue solution
School or college laboratory resources

So, this is what I know so far...

Enzymes become denatured when the temperature goes beyond the optimum temperature for the enzyme. [Is this investigation related to enzymes?]

If the methylene blue remains blue then respiration is not taking place, therefore the yeast is dead.

It's possible to distinguish between living and dead cells by examining them microscopically. [However I cannot tell which are blue and which are stained very clearly]

This is my method... however I don't think it is very good. [Please you could help me by stating any mistakes or how I could improve it. Thanks]

Place five boiling tubes in a test tube rack
Label boiling tubes 1 - 5
Add 5ml of yeast suspension to each tube
Heat tube 1 in a water bath at 40°C, tube 2 in a water bath at 50°C, tube 3 in a water bath at 60°C, tube 4 in a water bath at 70°C and tube 5 in a water bath at 80°C. [DO I NEED A CONTROL? AT ROOM TEMP?]
Leave tubes in water bath for 5 mins so they reach the temperature required
Now add 1 drop of methylene blue and 5ml of glucose solution to each boiling tube, gently shake, and leave for a further 10 minutes in appropriate water bath as stated previously [IS 10MINS A REASONABLE TIME]
After 10 mins removes tubes from water baths and gently shake then observe for a colour change
Use a pipette to put a drop of each mixture onto separate microscope slides
Study each slide carefully to observe whether the yeast cells are blue (dead) or colourless (alive)
Repeat the investigation to ensure it is a fair test

What do you think? Any more detail I should add?

Would you suggest I mention using a haemocytometer slide?

Finally, what IS the lowest temperature that kills all the yeast cells in a suspension of either dried or fresh baker's yeast. My Biology teacher keeps telling me to use the internet to find out but I can't seem to get an exact figure!!! Any other information you think I may find helpful would be greatly appreciated.

I'm also looking for some info on the web as I have to hand in my work on Monday the 23rd April.

I had a quick scan through your message, and have written down some pointers with my explanations to them.

You were correct in saying that the practical is based/linked to the activity of enzymes, namely those involved in respiration.
Yeast respires anaerobically (my teacher dropped a hint to me that I should look into this a little more) while fermenting sugars to alcohol.
Methylene blue turns colourless when in the presence of enzymes involved in respiration.
state that before beginning experiment, you must take a starter culture and add methylene blue-observe + describe sample of yeast cells under microscope when alive-then overheat them to kill yeast (denature enzymes), take new sample and observe + describe cells under microscope. this shows that you would know what dead yeast cells with methylene blue look like and aren't just making a presumption when you come to observe the results

As far as the different boiling tubes are concerned (my opinion):
-Best to raise the temp' of the water baths steadily from room temp' up to desired temperatures, as a rapid increase can overwhelm enzymes and denature them - producing unreliable results.
-It would be better to use diff' temp's that are closer together (every 5.c), based on your prediction of the lowest temperature that the yeast will be killed. In doing so you are provided with much more accurate results.
-Once you have a rough range in which the yeast is killed (example = between 45.c - 50.c) repeat the experiment again with temperatures within the range you have obtained, repeat one/two more times with smaller and smaller ranges of temperatures (like titration).

this planning exercise is a bit of a beast then.
basically i fink u gota use a colorimeter cos thats the only way to get nice statistical results rather than observing what you can see in teh cells under a microscope.
so u'd heat ur yeast, add ur glucose/sucrose and ur methylene blu and then put sum oil over the top cos its really the presence of oxygen which turns the indicator blue, then put it in a colorimter with blue filter paper and it shud tell u how much lights being detected, i.e. lots of the yeast is alive and nowt if the yeast is dead.
but does any1 hav a better suggestion?
and dya rekon we'd all b screwed if examners found out about this?

oo and also, just summit i thought which was kinda weird, it ses; "rapid temperature change tends to kill the cells". which makes me believe that the enzymes will be denatured at a specific temperature, say about 60, and also, they will be denatured if thers a rapid change say from 30 to 50 in a few seconds